Examined Japanese patent publications JP-B-3-12307 and JP-B-3-12308 (U.S. Pat. No. 4,629,676 and European Patent Publication No. 0174634) disclose methods comprising the steps of subjecting photosensitive materials containing silver halides, reducing agents and polymerizable compounds to image exposure; and developing the silver halides, thereby polymerizing the polymerizable compounds in the image to form polymer images. In these methods, polymerization is initiated by oxidant radicals of the reducing agents which have reduced the silver halides. (The radicals may be radicals produced by decomposition of oxidants of the reducing agents and are hereinafter referred to as "radicals"). More specifically, the photosensitive materials are subjected to heat development, thereby developing the silver halides and forming hardened images of the polymerizable compounds. The image forming methods described above can also be applied to the production of printing forms.
Photosensitive materials suitable for the production of printing forms are described in unexamined Japanese patent applications JP-A-5-249667 (U.S. Pat. No. 5,122,443 and European Patent Publication No. 0426192), JP-A-4-191856 (U.S. Pat. No. 5,290,659), JP-A-5-142775, JP-A-6-27652 and JP-A-5-107764. These documents disclose the use of crosslinking polymers in addition to or in place of the polymerizable compounds. In the methods for producing printing forms, the polymerizable compounds or the crosslinking polymers are hardened by the above-mentioned heat treatment. Then, unhardened portions are removed by using eluents. The remaining hardened portions are used as images for the printing forms.
In the printing forms, aluminum plates are generally used as supports. In the photosensitive materials used in the forms, the photosensitive layers containing silver halides and the hardenable layers containing polymerizable compounds or crosslinking polymers are separately provided in many cases.
When images are formed on the printing forms using the aluminum supports and photosensitive hardenable layers containing the silver halides; the reducing agents, the polymerizable compounds and color image forming materials, it becomes necessary to use a heat treatment device.
A conventional heat treatment device is shown in FIG. 5. Referring to FIG. 5, a photosensitive planographic printing form 114 is covered with a continuous belt 138 to hold the printing form 114 between the belt 138 and a heating plate 130. As the belt moves, the printing form is brought into sliding contact with the heating plate 130 and is heated by the heating plate 130 at the same time. The surface of the heating plate is curved in a circular arc, and the form 114 is formed thin enough to be curved along this arc. Tension is imparted to the continuous belt 138 to force the printing form 114 towards the heating plate 130. Transfer rollers 122 and 124 are mounted before and after the heating plate 130 with respect to the transfer direction A. The roller 122 guides the printing form 114 between the heating plate 130 and the continuous belt 138, and the transfer roller 124 guides the form out of the heating area.
When the printing form 114 is between the heating plate 130 and the continuous belt 138, the form is transferred by movement of the continuous belt 138 which is moving in the direction D. During the transfer, the form is heated as it is brought into sliding contact with the heating plate 130. Tension is imparted to the continuous belt 138 by means of the curvature 130B of the heating plate surface, thereby forcing the printing form towards the heating plate and resulting in close contact of the printing form with the heating plate. Thus, the printing form is heated throughout. The photosensitive material is heated at a temperature between 70.degree. and 200.degree. C.
When the heat treatment device shown in FIG. 5 is used for heat development, a metal support of a base on the printing form is expanded by this heating, causing slight distortion in the form. This results in incomplete contact with the heating plate 130. Moreover, when the photosensitive material is heat developed, heat energy supplied to the photosensitive material is decreased due to the distortion and incomplete contact. This causes a reduction in heat development efficiency. As a result, a polymerized hardened image is deficient at the point of deformation, or the degree of hardening is decreased and lowers the image strength. Accordingly, the press's usefulness is partially deteriorated. Furthermore, when attempts are made to form minute dots having a halftone dot area rate of 2% at a halftone dot number of 200 lines per inch, the inefficiency of heat development at the portions of incomplete contact cause insufficient hardening. Therefore, the minute halftone dots formed at these areas have a smaller size compared with the dots formed at a complete contact portion, and the resulting color tone is different. Thus, the image's uniformity deteriorates, and an objective for using the printing form is not satisfied.
Printing forms are usually large, and therefore incomplete contact due to heat expansion is liable to take place. This is one deficiency in the conventional art. In addition, because image density depends on the proper formation of minute halftone dots, it is very important to make the printing forms with uniform minute halftone dots.